Wood Movement Across Grain
Calculator

Solid wood keeps expanding and contracting with seasonal humidity changes for its entire life, so a tabletop or door panel built without allowing for that movement will eventually crack, cup, or blow out its joints when the seasons change.

Radial movement (measured across the growth rings, roughly quartersawn orientation) is typically about half of tangential movement (measured parallel to the growth rings, roughly flatsawn orientation), which is why quartersawn boards are more dimensionally stable than flatsawn boards of the same species.

Wood movement is driven by the change in equilibrium moisture content between two humidity conditions, not by humidity alone. A board built at 40% relative humidity and later exposed to 65% will expand; the size of that expansion depends on the gap between the two values and the wood's movement coefficient.

A common rule of thumb is roughly 1/4 inch of total movement allowance per foot of width for flatsawn wood in a typical seasonal humidity swing, but the precise figure depends on species and your local climate extremes, which is exactly what this calculator is meant to refine.

Rigid joinery like glue along the entire width of a panel, or screws through slotted holes that weren't actually free to slide, prevents the wood from moving and forces the stress into a crack instead. The fix is joinery that anchors the center and lets the edges float.